The purpose of this project is to biochemically and immunologically characterize retroviruses in order to understand the mechanisms by which these viruses induce cancer in their natural hosts. The role of these viruses in the etiology of human cancers is also under study. Studies currently in progress are the following: 1) biochemical characterization of replication-defective mammalian transforming viruses; 2) biochemical and immunological characterization of new isolates of retroviruses; 3) search for the presence of retroviral genes and gene products in human tumors.